This invention relates generally to electronic devices that use thermionic emission of electrons and, more particularly, to heater assemblies for heating cathodes to produce the thermionic emitted electrons.
As is known in the art, vacuum devices such as travelling wave tubes generally include a cathode which is heated to produce thermionically emitted electrons. Generally, the cathode is indirectly heated by use of a heater assembly which houses a filament. The filament is supplied a current to raise the temperature of the filament to a temperature in the range of at least 900.degree. C. to 1200.degree. C. The filament in the heater assembly provides the thermal energy required to raise the temperature of the cathode electrode to provide sufficient electron emission from the cathode to power the tube.
The heater assembly generally includes a filament wire which is coiled and is maintained in a position relative to the cathode throughout the operating life of the microwave tube. One approach to providing such heater assemblies is to provide a coiled filament wire supported by a dielectric potting. Generally, the dielectric used for the potting must be a relatively refractory material such as a ceramic in order to withstand the relatively high temperatures typically provided by the filament electrode. Since thermal transfer properties between the heater filament and the cathode are a critical characteristic to determine overall thermionic emission of electrons, the physical arrangement of the heater and the cathode must remain substantially constant over the operating life of the tube. Any variation in the position of the heater filament with respect to the cathode will cause a concomitant change in the temperature in the emitting surface of the cathode and thus a change in the rate of electron emission from the surface.
Electron emission from such a surface is very sensitive to temperature variations. Further, the cathode heater assemblies are subject to rapid changes in temperature which can cause failure of the assembly by cracking of the potting material. Moreover, in many applications of these tubes, such as in airborne applications the tubes are subjected to high levels of mechanical vibration and mechanical shock which likewise can have adverse affects on the potting material and can cause failure of the heater.
In order to provide a suitable potting for tubes presently used, the approach generally used is to provide a machined sleeve of a refractory type of metal to which is attached or formed a "cathode button" or the cathode electrode from which thermionically emitted electrons are provided. The sleeve and button in combination provide a mold into which an aqueous slurry of a refractory oxide such as aluminum oxide powder is introduced to encapsulate a coiled filament wire which is disposed in the mold. The aluminum oxide powder or other refractory oxide powders used in the slurry are characterized as having relatively irregular, random shapes and large particle sizes. The slurry is introduced to the mold provided by the sleeve and cathode button. At this juncture, the slurry has a green or prefire density of about 40% of theoretical density (T.D.). The slurry in the mold is fired to sinter the aluminum oxide or other refractory oxide into a solid mass. With such a low green state density, a large degree of shrinkage occurs during the sintering process as there is a concomitant reduction in the volume of the slurry material as water is released from the slurry material and the aluminum oxide coalesces into a consolidated or more densified mass. The reduction in volume which accompanies the step of densifying the mass requires the addition of more slurry to the mold and repeating the high temperature firing or sintering until the potting is built up to its final height. That is, the approach requires additional reworking of the potting until the final height of the potting is provided. Generally, sintered potted assemblies do not attain a final density of more than 80% of T.D. Often the density is in the range of 70% to 75% of T.D.
Several problems are present with this approach. The first problem is a consideration of cost. Since multiple slurry addition and firing cycles are generally required to provide a suitable potting for the heater, the additional cycles increase the cost of processing of the potted assembly. Further, due to the relatively large shrinkage or decrease in volume of the potting between its "green" or prefired stage and the potting after having been fired or sintered, defects in the material of the potting often occur. Such defects can include cracks and voids. Often these cracks and voids occur in areas of the potting which are not visible or accessible and thus cannot be reworked. These latter problems contribute to relatively low yields for these structures, as well as, a potting having relatively low mechanical strength, relatively low density, and less than ideal thermal transfer characteristics.